Checking entries before submitting

Is there a simple way to check user entries before the form gets submitted?

Ideally, I would like to check the values that are about to be submitted, and if there is an error I would like to print out some helpful error message, otherwise I want everything to proceed as normal.

If someone could provide some sample code on how to do this, I would be grateful.

Note, I've tried the following:



get "Name==$_put{To}", "main"; if ($Name eq "") {

out<<EOF;
Can't find the person who is getting the credit. Please check the spelling of the name.
EOF


}


But the problem is that the error comes up when the form is first visited.